/6 Possibilities

Here's a wild one for you. http://www.youtube.com/watch?v=8d-qqeK0B-I

When this gets wound up, it sounds like a V-12.

Two places I recommend you check. YouTube for Slant Six Exhaust for sonic expectations. http://www.slantsix.org for possibilities of what can be done with the leaning tower of power.

Given you have made some choices already. Recommend milling the head .050. No gas problems and you won't have to go to shorter pushrods. If a pre72 engine, recommend hardened valve seats.

To get a meaner sound, suggest changing the cam. Just enlarging the pipe produces a deeper tone. With the low restriction muffler, a few more mechanical noises may creep through. There's a group buy for a Sig Erson over as slantsix.org that is generating a bit of interest. Also, CompCams & Mopar Performance each make a pair of cams for the engine.